Moulded article made of polytetrafluoroethylene

ABSTRACT

Bicomponent moulded article made of polytetrafluoroethylene (PTFE) which, in the manner of the side-by-side type, is composed of two PTFE components which shrink under the effect of heat and differ in their hot-air shrinkage by at least 1%. A preferred embodiment is in the form of a fibre which can be crimped under heat treatment. (FIG. 1)

The invention relates to a moulded article made ofpolytetrafluoroethylene (PTFE) which is shaped as a film, tape or fibre,and a process for the production thereof.

Because of its thermal stability and its chemical inertness, PTFE is avalued material. AT-B 370,674 discloses monoaxially stretched films madeof sintered PTFE which in the stretch direction have values for strengthof between 50 N/mm² and 140 N/mm². These films are produced by firstpressing PTFE powder to form a cylindrical moulded article. Subsequentlythe moulded article is sintered, whereupon films are peeled off, heatedto temperatures of at least 327° C. and stretched.

GB-A-2 025 835 describes the production of porous PTFE moulded articlesby the paste-extrusion process, a pasty composition, containingessentially PTFE powder and a lubricant, being forced through dies,whereupon the lubricant is removed by drying. Then the moulded articleis heated to above the crystallite melting point of the PTFE (327° C.)and stretched during heating.

AT-B 391,473 discloses the production of a monoaxially stretched mouldedarticle made of PTFE, a pasty PTFE powder-containing composition beingcontinually processed to form a moulding which is led over a pluralityof rollers or rolls and is heated and stretched, the moulding, beforeapplication of the stretching, being heated to a temperature between327° and 450° C., preferably between 350° and 390° C., sintered duringthis and then stretched. This process permits the production of amonoaxially stretched moulded article from PTFE with strength values inthe stretching direction of at least 500 N/mm² and a density of between1.80 and 2.30 g/cm³.

In order to increase the range of application of PTFE, it would bedesirable for crimped fibres made of PTFE to be available. The inventiontherefore has the object of making available PTFE fibres having a stableand permanent crimp.

This aim is achieved according to the invention by means of abicomponent moulded article of polytetrafluoroethylene (PTFE) which iscomposed, in the manner of the side-by-side type, of two PTFE componentswhich shrink under the effect of heat and differ in their hot-airshrinkage by at least 1%. If, for example, a fibre of this kind isheated to temperatures above 200° C., it crimps insofar as it can shrinkfreely during the heat treatment.

It is known from the Ullmann "Encyklopadie der technischen Chemie", 4thedition, volume 11, pages 283-284, Verlag Chemie that nylon fibres canbe provided with a crimp by bicomponent spinning. According to thisprocess, two polymer melts with different properties are spun from a diehole to form a filament. In this there are in principle twopossibilities for arranging the two polymers in the fibre: a core-sheatharrangement and a side-by-side arrangement. In the first arrangement,one polymer surrounds the other in the manner of a sheath and in thelatter the fibres consist of two imaginary half-cylinders which arejoined to one another via their planes of section. However, it is notpossible to produce PTFE fibres having a stable crimp using thisprocess.

The invention is based on the recognition that a bicomponent filmconsisting of two or more PTFE layers which differ in their shrinkproperties can be processed in a simple manner to form a crudebicomponent fibre, which can be provided with a stable crimp by heatingto a temperature above 200° C.

A preferred embodiment of the bicomponent moulded article according tothe invention is therefore designed as a film, tape or fibre. The fibreis expediently fibrillated.

The invention relates to a moulded article made of PTFE which isobtained by heat treatment of the aforementioned bicomponent mouldedarticle.

A preferred embodiment of the moulded article is characterized in thatit is in the form of a crimped fibre. This fibre can be processed in aconventional manner to form a needled felt consisting exclusively ofPTFE. The crimped PTFE fibre has strength values of at least 300 N/mm².

It has been found that the carding properties of the fibres according tothe invention are particularly good and the fibres are very especiallyreadily suitable for forming non-woven fabrics and felts if fibres areused which are crimped and fibrillated. Using these fibres which arebranched by the fibrillation, a pure PTFE felt can be produced withoutthe use of auxiliary means. This is of great advantage since thesefelts, when used as filters, have a lower increase in pressuredifference than felts made of matrix-spun fibres. The felts according tothe invention can be readily used in medical technology.

The invention further relates to a process for producing a bicomponentfilm from PTFE which consists in two PTFE powder-containing compositionsbeing processed to form a moulding of the side-by-side type which isrolled out to form a flat bicomponent moulded article which is sinteredand subsequently stretched, with the proviso that PTFE powders are usedwhich, processed to form a film, shrink to different extents under theeffect of heat and differ in their hot-air shrinkage by at least 1%. Itis also conceivable to process more than two PTFE powder-containingcompositions to form a moulding of the side-by-side type.

A preferred modification of the process according to the inventionconsists in cutting the flat bicomponent moulded article to form tapeswhich are sintered, subsequently stretched and, if required, subjectedto a fibrillation treatment.

The invention is explained in greater detail with the aid of theaccompanying drawing. FIG. 1 shows diagrammatically the formation andstructure of a flat bicomponent moulded article of the side-by-sidetype, and FIG. 2 shows diagrammatically the production of a crimpedfibre made of PTFE.

According to FIG. 1, two PTFE polymers A and B which differ in theirhot-air shrinkage are first mixed with lubricants and formed into themouldings (cylinders). These cylinders are subsequently axially splitalong their length into two halves and in each case one half A and Bused jointly in an extruder and extruded to form a strand. One half ofthe strand thus consists of PTFE A and the other half of PTFE B. Thisstrand is then, in the next stage, led through a calender and rolled outto form a film. In this procedure, care should be taken that the strandruns in between the rolls such that the imaginary separating line of thetwo material halves lies parallel to the roll nip. Only in this manneris it ensured that a bicomponent film is actually produced. Followingthe calendering, the film is dried as usual and thus freed of lubricant.

The dried film is now continuously fed to the sintering and stretchingsystem shown diagrammatically in FIG. 2. It consists essentially of adelivery element 1, a blade bar 2, two heated rolls 3', 3" and anunheated take-off element 4, the arrows in the drawing symbolizing thedirection of rotation of the heated rolls 3', 3" and the take-offdirection of the film.

In order to produce tapes and fibres, the dried film is advantageouslycut by the blade bar 2 as early as its passage through the deliveryelement 1.

The film transferred to the delivery element 1 is cut continuously intotapes by the blade bar 2, which tapes are guided further over the heatedrolls 3', 3" and are sintered on these. To this end, the tapes arepreferably laid in an "S" shape around the rolls in order to heat thetapes through from both sides. The rolls are heated to a temperature ofat least 327° C. The stretching takes place directly after the sinteringprocess. Takeoff is carried out by the unheated take-off element 4 whichis shown with a plurality of reels in FIG. 2. In order to producefibres, the tapes are subsequently laid over a needle roll, and arefibrillated. In order now additionally to activate the crimping, thefibre is heated without stressing to a temperature of at least 200° C.,that is to say shrunk freely.

The invention is explained in even greater detail by means of thefollowing examples.

EXAMPLE 1

PTFE powders of the types "Algoflon" DF 200 (manufacturer: Ausimont SpA)with a hot-air shrinkage of 4.5% and "Teflon" 3579 (manufacturer Dupont)with a hot-air shrinkage of 3.0% are separately treated with in eachcase 20% of lubricant with a boiling range of between 186° and 214° C.and are separately pressed to form cylinders. These cylinders aresubsequently separated in the axial direction, exactly in the centre.One half, in each case, of the two different polymers are then used inthe main pressing cylinder and pressed to form a round strand. Thisstrand is subsequently rolled out by means of a two-roll calender toform a 0.1 mm thick film in such a manner that, over the entire width ofthe film, one half of the thickness of the film consists of "Algoflon"DF 200 and the other half consists of "Teflon" 3579. The film is dried,sintered at a temperature of 380° C. and stretched at this temperaturein a ratio of 1:11. The film thus obtained has a thickness of 20 μm andis split into filaments by means of fibrillating rolls and cut using acutting machine into a staple fibre having a length of 80 mm. The staplefibre obtained are further conditioned in an oven at a temperature of250° C. for 30 minutes. The two PTFE polymers shrink to differentextents during this and a crimping of 45% results. The strength of thisfibre is 495 N/mm² at an elongation of 8%.

EXAMPLE 2

A film having a thickness of 50 μm is produced, stretched andsubsequently fibrillated as described in Example 1. The stretching ratiois 1:12 and the film thickness obtained is 9.5 μm. The shrinking andcrimping take place at a temperature of 300° C. The crimping obtained is65%, the strength is 530 N/mm² at an elongation of 6.5%.

EXAMPLE 3

According to Example 1, "Teflon" 3579 and "Polyflon" 104 UF(manufacturer: Daikin) with a hot-air shrinkage of 5.5% are mixed withlubricant and formed into a film having a thickness of 75 μm. Thestretching takes place at a temperature of 365° C. and a ratio of 1:10.The 15 μm thick film obtained is then fibrillated and shrunk in a towerheated to 300° C. and thus crimped. After the crimping thus carried out,the strand is cut into fibres. The crimping is 60%. The strength is 427N/mm² at an elongation of 9%.

EXAMPLE 4

PTFE powder is mixed according to Example 3 and subsequently pressed toform a cylinder by filling one half of the press mould with one powderand the other half with the other powder in equal quantities. Thetwo-component cylinder produced in this manner is processed in themanner described in Example 3 to form a fibre. The shrinkage takes placeat 310° C., the crimping is 62%. The strength of the crimped fibres is405 N/mm² at an elongation of 10%.

EXAMPLE 5

As described in Example 4, "Algoflon" DF 200 and "Hostaflon" TF 2029(manufacturer: Hoechst), the latter possessing a hot-air shrinkage of2.5%, are filled together into the mould, pressed to form a strand,further formed into a film and, under the conditions described inExample 1, stretched, fibrillated and thermally shrunk. The strength ofthe fibre obtained is 480 N/mm² at an elongation of 8%; the crimping is50%.

We claim:
 1. A molded article which comprises:a) a first component whichcomprises polytetrafluoroethylene; and b) a second component whichcomprises polytetrafluoroethylene, wherein the first and secondcomponents are joined to one another in side-by-side relationship,wherein the first and second components differ in their hot-airshrinkage at a temperature above 200° C. by at least 1% and wherein themolded article is a fiber.
 2. A molded article according to claim 1 thewherein said fiber is fibrillated.
 3. A process for preparing an articleof polytetrafluoroethylene, comprising the steps of:a) processing twopolytetrafluoroethylene powder-containing compositions to form amolding, wherein the molding has a first and secondpolytetrafluoroethylene component joined to each other in side-by-siderelationship, wherein the polytetrafluoroethylene components differ intheir hot air shrinkage at a temperature above 200° C. by at least 1%;b) rolling out the molding to form a flat molded article; c) cutting theflat molded article into tapes; d) sintering the tapes; and e)stretching the tapes.
 4. A process for preparing an article ofpolytetrafluoroethylene, comprising the steps of:a) processing twopolytetrafluoroethylene powder-containing compositions to form amolding, wherein the molding has a first and secondpolytetrafluoroethylene component joined to each other in side-by-siderelationship, wherein the polytetrafluoroethylene components differ intheir hot air shrinkage at a temperature above 200° C. by at least 1%;b) rolling out the molding to form a flat molded article; c) cutting theflat molded article into tapes; d) sintering tapes article; e)stretching the tapes; and f) subjecting the tapes to a fibrillationtreatment.